Fuel cell electrode assembly with selective catalyst loading

ABSTRACT

A membrane electrode assembly is provided. More particularly, an assembly is provided which includes a pair of electrodes and an ion exchange membrane having opposed major surfaces positioned between the electrodes, at least one of the electrodes being formed of a sheet of a compressed mass of expanded graphite particles having a plurality of transverse fluid channels passing through the sheet between first and second opposed surfaces of the sheet, one of opposed surfaces abutting the ion exchange membrane. At least one of the surfaces of the ion exchange membrane has a catalyst metal selectively loaded thereon.

TECHNICAL FIELD

This invention relates to an electrode assembly for an electrochemicalfuel cell that includes an article formed of flexible graphite sheetthat is fluid permeable and has enhanced isotropy with respect tothermal and electrical conductivity. The electrode assembly alsoincludes an ion exchange membrane having catalyst metal selectivelyloaded thereon.

BACKGROUND OF THE INVENTION

An ion exchange membrane fuel cell, more specifically a proton exchangemembrane (PEM) fuel cell, produces electricity through the chemicalreaction of hydrogen and oxygen in the air. Within the fuel cell, ananode and cathode surround a polymer electrolyte. A catalyst materialstimulates hydrogen molecules to split into hydrogen atoms and throughan internal load at the membrane the atoms each split into a proton andan electron. The electrons generated are utilized as electrical energy.The protons migrate through the electrolyte and combine with oxygen fromthe air and electrons to form water.

A PEM fuel cell is advantageously formed of a membrane electrodeassembly sandwiched between two graphite flow field plates.Conventionally, the membrane electrode assembly consists of electrodes(anode and cathode) with a thin layer of a catalyst material,particularly platinum or a platinum group metal coated on graphite orcarbon particles, bonded to either side of a proton exchange membranedisposed between the electrodes. In operation, hydrogen flows throughchannels in one of the flow field plates to the anode, where thecatalyst promotes its separation into hydrogen atoms and thereafter intoprotons and electrons. Air flows through the channels in the other flowfield plate to the cathode, where the oxygen in the air attracts theprotons through the proton exchange membrane and the electrons throughthe circuit, which join to form water. Since electrons cannot passthrough the PEM, they travel through the anode, through a circuit inwhich the electricity is utilized, and back to the cathode. The airstream on the cathode side removes the water formed by combination ofthe hydrogen and oxygen. Combinations of such fuel cells are used in afuel cell stack to provide the desired electrical power.

One limiting factor to the more widespread use of PEM fuel cells is thecost of the catalyst material. Metals such as platinum and the otherplatinum group metals are relatively expensive, especially as comparedto the other elements of the cell, such as the graphite flow fieldplates. In conventional fuel cells, the catalyst material is bonded tothe PEM or electrodes in a non-selective manner. That is, the catalystmaterial is distributed relatively uniformly on the opposed surfaces ofthe PEM, with result that catalyst deployment is not as efficient aspossible.

Graphites are made up of layer planes of hexagonal arrays or networks ofcarbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another. The substantiallyflat, parallel equidistant sheets or layers of carbon atoms, usuallyreferred to as basal planes, are linked or bonded together and groupsthereof are arranged in crystallites. Highly ordered graphites consistof crystallites of considerable size: the crystallites being highlyaligned or oriented with respect to each other and having well orderedcarbon layers. In other words, highly ordered graphites have a highdegree of preferred crystallite orientation. It should be noted thatgraphites possess anisotropic structures and thus exhibit or possessmany properties that are highly directional e.g. thermal and electricalconductivity and fluid diffusion. Briefly, graphites may becharacterized as laminated structures of carbon, that is, structuresconsisting of superposed layers or laminae of carbon atoms joinedtogether by weak van der Waals forces. In considering the graphitestructure, two axes or directions are usually noted, to wit, the “c”axis or direction and the “a” axes or directions. For simplicity, the“c” axis or direction may be considered as the direction perpendicularto the carbon layers. The “a” axes or directions may be considered asthe directions parallel to the carbon layers or the directionsperpendicular to the “c” direction. The graphites suitable formanufacturing flexible graphite possess a very high degree oforientation.

As noted above, the bonding forces holding the parallel layers of carbonatoms together are only weak van der Waals forces. Natural graphites canbe treated so that the spacing between the superposed carbon layers orlaminae can be appreciably opened up so as to provide a marked expansionin the direction perpendicular to the layers, that is, in the “c”direction and thus form an expanded or intumesced graphite structure inwhich the laminar character of the carbon layers is substantiallyretained.

Graphite flake which has been greatly expanded and more particularlyexpanded so as to have a final thickness or “c” direction dimensionwhich is as much as about 80 or more times the original “c” directiondimension can be formed without the use of a binder into cohesive orintegrated flexible graphite sheets of expanded graphite, e.g. webs,papers, strips, tapes, or the like. The formation of graphite particleswhich have been expanded to have a final thickness or “c” dimensionwhich is as much as about 80 times the original “c” direction dimensioninto integrated flexible sheets by compression, without the use of anybinding material is believed to be possible due to the excellentmechanical interlocking, or cohesion which is achieved between thevoluminously expanded graphite particles.

In addition to flexibility, the sheet material, as noted above, has alsobeen found to possess a high degree of anisotropy with respect tothermal and electrical conductivity and fluid diffusion, comparable tothe natural graphite starting material due to orientation of theexpanded graphite particles substantially parallel to the opposed facesof the sheet resulting from very high compression, e.g. roll pressing.Sheet material thus produced has excellent flexibility, good strengthand a very high degree of orientation.

Briefly, the process of producing flexible, binderless anisotropicgraphite sheet material, e.g. web, paper, strip, tape, foil, mat, or thelike, comprises compressing or compacting under a predetermined load andin the absence of a binder, expanded graphite particles which have a “c”direction dimension which is as much as about 80 or more times that ofthe original particles so as to form a substantially flat, flexible,integrated graphite sheet. The expanded graphite particles thatgenerally are worm-like or vermiform in appearance, once compressed,will maintain the compression set and alignment with the opposed majorsurfaces of the sheet. The density and thickness of the sheet materialcan be varied by controlling the degree of compression. The density ofthe sheet material can be within the range of from about 0.08 g/cc toabout 2.0 g/cc. The flexible graphite sheet material exhibits anappreciable degree of anisotropy due to the alignment of graphiteparticles parallel to the major opposed, parallel surfaces of the sheet,with the degree of anisotropy increasing upon roll pressing of the sheetmaterial to increased density. In roll pressed anisotropic sheetmaterial, the thickness, i.e. the direction perpendicular to theopposed, parallel sheet surfaces comprises the “c” direction and thedirections ranging along the length and width, i.e. along or parallel tothe opposed, major surfaces comprises the “a” directions and thethermal, electrical and fluid diffusion properties of the sheet are verydifferent, by orders of magnitude, for the “c” and “a” directions.

This very considerable difference in properties, i.e. anisotropy, whichis directionally dependent, can be disadvantageous in some applications.For example, in gasket applications where flexible graphite sheet isused as the gasket material and in use is held tightly between metalsurfaces, the diffusion of fluid, e.g. gases or liquids, occurs morereadily parallel to and between the major surfaces of the flexiblegraphite sheet. It would, in most instances, provide for greater gasketperformance, if the resistance to fluid flow parallel to the majorsurfaces of the graphite sheet (“a” direction) were increased, even atthe expense of reduced resistance to fluid diffusion flow transverse tothe major faces of the graphite sheet (“c” direction). With respect toelectrical properties, the resistivity of anisotropic flexible graphitesheet is high in the direction transverse to the major surfaces (“c”direction) of the flexible graphite sheet, and very substantially lessin the direction parallel to and between the major faces of the flexiblegraphite sheet (“a” direction). In applications such as fluid flow fieldplates for fuel cells and seals for fuel cells, it would be of advantageif the electrical resistance transverse to the major surfaces of theflexible graphite sheet (“c” direction) were decreased, even at theexpense of an increase in electrical resistivity in the directionparallel to the major faces of the flexible graphite sheet (“a”direction).

With respect to thermal properties, the thermal conductivity of aflexible graphite sheet in a direction parallel to the upper and lowersurfaces of the flexible graphite sheet is relatively high, while it isrelatively very low in the “c” direction transverse to the upper andlower surfaces.

The foregoing situations are accommodated by the present invention.

SUMMARY OF THE INVENTION

The present invention provides a membrane electrode assembly for anelectrochemical fuel cell comprising a pair of electrodes and an ionexchange membrane having opposed major surfaces positioned between theelectrodes. At least one of the electrodes is formed of a sheet of acompressed mass of expanded graphite particles having a plurality oftransverse fluid channels passing through the sheet between first andsecond opposed surfaces of the sheet, one of the opposed surfacesabutting the ion exchange membrane. The transverse fluid channels areformed by mechanically impacting an opposed surface of the graphitesheet to displace graphite within the sheet at predetermined locationsto provide a channel pattern. The transverse fluid channels areadjacently positioned and separated by walls of compressed expandedgraphite. The ion exchange membrane has catalyst metal selectivelyloaded on it in a manner complementary to the channel pattern of atleast one of the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a transversely permeable sheet of flexiblegraphite having transverse channels in accordance with the presentinvention;

FIG. 2 is a side elevation view in section of the sheet of FIG. 1;

FIGS. 2(A), (B), (C), (D) show various suitable flat-endedconfigurations for transverse channels in accordance with the presentinvention;

FIGS. 3, 3(A) shows a mechanism for making the article of FIG. 1;

FIG. 4 shows an enlarged sketch of an elevation view of the orientedexpanded graphite particles of prior art flexible graphite sheetmaterial;

FIG. 5 is a sketch of an enlarged elevation view of an article formed offlexible graphite sheet in accordance with the present invention;

FIGS. 6, 7 and 7(A) show a fluid permeable electrode assembly whichincludes a transversely permeable article in accordance with the presentinvention; and

FIG. 8 is a photograph at 100× (original magnification) corresponding toa portion of the side elevation view sketch of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Graphite is a crystalline form of carbon comprising atoms covalentlybonded in flat layered planes with weaker bonds between the planes. Bytreating particles of graphite, such as natural graphite flake, with anintercalant of, e.g. a solution of sulfuric and nitric acid, the crystalstructure of the graphite reacts to form a compound of graphite and theintercalant. The treated particles of graphite are hereafter referred toas “particles of intercalated graphite”. Upon exposure to hightemperature, the intercalant within the graphite decomposes andvolatilizes, causing the particles of intercalated graphite to expand indimension as much as about 80 or more times its original volume in anaccordion-like fashion in the “c” direction, i.e. in the directionperpendicular to the crystalline planes of the graphite. The exfoliatedgraphite particles are vermiform in appearance, and are thereforecommonly referred to as worms. The worms may be compressed together intoflexible sheets that, unlike the original graphite flakes, can be formedand cut into various shapes and provided with small transverse openingsby deforming mechanical impact.

Graphite starting materials suitable for use in the present inventioninclude highly graphitic carbonaceous materials capable of reversiblyintercalating alkali metals. These highly graphitic carbonaceousmaterials have a degree of graphitization above about 0.80 and mostpreferably about 1.0. As used in this disclosure, the term “degree ofgraphitization” refers to the value g according to the formula:$g = \frac{3.45 - {d\quad (002)}}{0.095}$

where d(002) is the spacing between the graphitic layers of the carbonsin the crystal structure measured in Angstrom units. The spacing dbetween graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous anode materials include synthetic graphites and naturalgraphites from various sources, as well as other carbonaceous materialssuch as petroleum cokes heat treated at temperatures above 2500° C.,carbons prepared by chemical vapor deposition or pyrolysis ofhydrocarbons and the like.

The graphite starting materials used in the present invention maycontain non-carbon components so long as the crystal structure of thestarting materials maintains the required degree of graphitization.Generally, any carbon-containing material, the crystal structure ofwhich possesses the required degree of graphitization, is suitable foruse with the present invention. Such graphite preferably has an ashcontent of less than six weight percent.

A common method for manufacturing graphite sheet is described by Shaneet al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 50 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solutions maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

The quantity of intercalation solution may range from about 20 to 150parts of solution by weight per 100 parts by weight of graphite flakes(pph) and more typically about 50 to 120 pph. After the flakes areintercalated, any excess solution is drained from the flakes and theflakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 to 50 parts ofsolution per hundred parts of graphite by weight (pph) which permits thewashing step to be eliminated as taught and described in U.S. Pat. No.4,895,713, the disclosure of which is also herein incorporated byreference. The thus treated particles of graphite are sometimes referredto as “particles of intercalated graphite”. Upon exposure to hightemperature, e.g. about 700° C. to 1000° C. and higher, the particles ofintercalated graphite expand as much as about 80 to 1000 or more timesits original volume in an accordion-like fashion in the c-direction,i.e. in the direction perpendicular to the crystalline planes of theconstituent graphite particles. The expanded, i.e. exfoliated graphiteparticles are vermiform in appearance, and are therefore commonlyreferred to as worms. The worms may be compressed together into flexiblesheets that, unlike the original graphite flakes, can be formed and cutinto various shapes and provided with small transverse openings bydeforming mechanical impact as hereinafter described.

Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed, e.g. by roll-pressing, to athickness of about 0.075 mm to 3.75 mm and a density of about 0.1 to 1.5grams per cubic centimeter. From about 1.5-30% by weight of ceramicadditives, can be blended with the intercalated graphite flakes asdescribed in U.S. Pat. No. 5,902,762 (which is incorporated herein byreference) to provide enhanced resin impregnation in the final flexiblegraphite product. The additives include ceramic fiber particles having alength of about 0.15 to 1.5 millimeters. The width of the particles issuitably from about 0.04 to 0.004 mm. The ceramic fiber particles arenon-reactive and non-adhering to graphite and are stable at temperaturesup to about 1100° C., preferably about 1400° C. or higher. Suitableceramic fiber particles are formed of macerated quartz glass fibers,carbon and graphite fibers, zirconia, boron nitride, silicon carbide andmagnesia fibers, naturally occurring mineral fibers such as calciummetasilicate fibers, calcium aluminum silicate fibers, aluminum oxidefibers and the like.

The flexible graphite sheet can also, at times, be advantageouslytreated with resin and the absorbed resin, after curing, enhances themoisture resistance and handling strength, i.e. stiffness of theflexible graphite sheet. Suitable resin content is preferably about 20to 30% by weight, suitably up to about 60% by weight.

In the practice of this invention, the flexible graphite sheet isprovided with channels, which are preferably smooth-sided, and whichpass between the parallel, opposed surfaces of the flexible graphitesheet and are separated by walls of compressed expandable graphite. Itis the walls of the flexible graphite sheet that actually abut the ionexchange membrane, when the inventive flexible graphite sheet functionsas an electrode in an electrochemical fuel cell.

The channels preferably have openings on one of the opposed surfacesthat are larger than the openings in the other opposed surface. Thechannels can have different configurations, which can be formed, forinstance, using flat-ended protrusion elements of different shapes. Thesmooth flat-ends of the protrusion elements preferably ensuredeformation and complete displacement of graphite within the flexiblegraphite sheet, i.e. there are no rough or ragged edges or debrisresulting from the channel-forming impact. Preferred protrusion elementshave decreasing cross-section in the direction away from the pressingforce to provide larger channel openings on the side of the sheet thatis initially impacted. The development of smooth, unobstructed surfacessurrounding channel openings enables the free flow of fluid into andthrough smooth-sided channels. In a preferred embodiment, openings oneof the opposed surfaces are larger than the channel openings in theother opposed surface, e.g. from 1 to 200 times greater in area, andresult from the use of protrusion elements having converging sides.

The channels are formed in the flexible graphite sheet at a plurality oflocations by mechanical impact. Thus, a pattern of channels is formed inthe flexible graphite sheet. That pattern can be devised in order tocontrol, optimize or maximize fluid flow through the channels, asdesired. For instance, the pattern formed in the flexible graphite sheetcan be uniform about the sheet (i.e., the channels are relatively evenlydistributed on the sheet) or it can comprise selective placement of thechannels, as described, or it can comprise variations in channel densityor channel shape in order to, for instance, equalize fluid pressurealong the surface of the electrode when in use, as well as for otherpurposes which would be apparent to the skilled artisan.

The impact force is preferably delivered using a patterned roller,suitably controlled to provide well-formed perforations in the graphitesheet. In the course of impacting the flexible graphite sheet to formchannels, graphite is displaced within the sheet to disrupt and deformthe parallel orientation of the expanded graphite particles. In effectthe displaced graphite is being “die-molded” by the sides of adjacentprotrusions and the smooth surface of the roller. This reduces theanisotropy in the flexible graphite sheet and thus increases theelectrical and thermal conductivity of the sheet in the directiontransverse to the opposed surfaces. A similar effect is achieved withfrusto-conical and parallel-sided peg-shaped flat-ended protrusions.

As noted above, the inventive membrane electrode assembly comprises anion exchange membrane sandwiched between two electrodes, at least one ofwhich is the above-described graphite sheet. A typical substrate for theion (or proton) exchange membrane is a porous material, such as a glasscloth or a polymeric material such as a porous polyolefin likepolyethylene or polypropylene. Preferably, for use in a commercialpractical electrochemical fuel cell, the substrate for the PEM should bebetween about 10 and 200 microns thick, with an average pore diameter ofabout 0.1 to about 1.0 microns and porosity of about 50 to 98%.Perfluorinated polymers, like polytetrafluoroethylene, are sometimespreferred. The substrate can then be impregnated to control propertiessuch as porosity. Styrene impregnants such as trifluorostyrene andsubstituted trifluorostyrenes have been suggested as particularlysuitable for use in fuel cell proton exchange membranes. One preferredimpregnant useful in the practice of the invention is available from IonPower Inc. under the tradename Liquione-1100; an especially preferredimpregnant is a perfluorinated polymer membrane sold under the tradenameNafion® by DuPont Company.

Suitable materials for use as the proton exchange membrane are describedin U.S. Pat. Nos. 5,773,480 and 5,834,523, the disclosures of each ofwhich are incorporated herein by reference.

In order to facilitate and/or enable the dissociation/associationreactions required for fuel cell operation, a catalyst metal is loadedon the two opposed major surfaces of the PEM. Most commonly, thecatalyst is a noble metal like platinum or a platinum group metal, oftenloaded on graphite or carbon particles. The catalyst can be loadeddirectly to the surface of the PEM, or a catalyst-loaded moiety, such asactivated carbon paper can be bonded to either surface of the PEM, aswould be familiar to the skilled artisan. Indeed, the catalyst can alsobe loaded on the inventive membrane electrode.

In operation, the fluid (i.e., either hydrogen gas or oxygen gas,depending on the “side” of the membrane electrode assembly in question)contacts the catalyst. In the case of hydrogen, on the anodic side ofthe assembly, the catalyst catalyzes the dissociation of the hydrogen toits constituent protons and electrons; the protons then migrate throughthe proton exchange membrane, and the electrons are then utilized aselectrical energy. In the case of oxygen, on the cathodic side of theassembly, the catalyst catalyzes the association of the protons andelectrons, with the oxygen, to form water.

In accordance with the present invention, distribution of the catalystmetal on the opposed surfaces of the PEM is controlled, or selected, byknown methods, such as “printing” processes, such that the catalystmetal is arrayed about the surface of the PEM at locations at or nearwhere the electrode contacts the PEM, i.e., where the desired reactionsoccur. In this way, catalyst is not present at locations where it willnot employed in the dissociation/association reactions necessary forfuel cell operation, and where, therefore, the catalyst would berelatively ineffective. It will be understood that the expression “thecatalyst metal is arrayed about the surface of the PEM” is also meant toinclude the situation where the metal is loaded on the electrode whereit abuts the PEM.

The catalyst is selectively loaded on the proton exchange membrane in apattern determined by the channel pattern of the electrode that facesthe particular PEM side. Most preferably, the catalyst is arrayed on thePEM at locations corresponding to the walls forming channels of thegraphite sheet electrode as described above (such as by loading thecatalyst metal on the electrode walls themselves), in order to maximizecatalyst effectiveness. In this way, less catalyst is needed for similarelectrical outputs from the fuel cell, resulting in significant savingsand other advantages, such as reduced catalyst recycle costs.

More particularly, in operation, the reaction catalyzed by the catalystmetal, that is, the dissociation of hydrogen molecules and atoms intoconstituent protons and electrons, and the re-association of the protonsand electrons, in combination with oxygen, into water, occurs at thepoint where the surfaces of the electrode meet (or abut), or are inelectrical contact with, the ion exchange membrane. It is at thislocation and, effectively, only at this location, where dissociatedelectrons can be conducted along the electrode and dissociated protonscan migrate across the membrane (and vice versa with respect to thecathodic side of the fuel cell). Thus, it is only at or near where thewalls of the channels formed in the inventive graphite sheet abut theion exchange membrane, where catalyst is selectively loaded inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1 and FIG. 2, a compressed mass of expandedgraphite particles, in the form of a flexible graphite sheet is shown at10. The flexible graphite sheet 10 is provided with channels 20, whichare preferably smooth-sided as indicated at 67 in FIGS. 5 and 8, andwhich pass between the opposed surfaces 30, 40 of flexible graphitesheet 10, and are separated by walls 3 of compressed expandablegraphite. The channels 20 preferably have openings 50 on one of theopposed surfaces 30 which are larger than the openings 60 in the otheropposed surface 40. The channels 20 can have different configurations asshown at 20′-20′″ in FIGS. 2(A), 2(B), 2(C), 2(d) which are formed usingflat-ended protrusion elements of different shapes as shown at 75, 175,275, 375 in FIGS. 2(A), 2(B), 2(C), 2(d) suitably formed of metal, e.g.steel and integral with and extending from the pressing roller 70 of theimpacting device shown in FIG. 3. The smooth flat-ends of the protrusionelements, shown at 77, 177, 277, 377, and the smooth bearing surface 73,of roller 70, and the smooth bearing surface 78 of roller 72 (oralternatively flat metal plate 79), ensure deformation and completedisplacement of graphite within the flexible graphite sheet, i.e. thereare no rough or ragged edges or debris resulting from thechannel-forming impact. Preferred protrusion elements have decreasingcross-section in the direction away from the pressing roller 70 toprovide larger channel openings on the side of the sheet that isinitially impacted. The development of smooth, unobstructed surfaces 63surrounding channel openings 60, enables the free flow of fluid into andthrough smooth-sided (at 67) channels 20.

In a preferred embodiment, openings on one of the opposed surfaces arelarger than the channel openings in the other opposed surface, e.g. from1 to 200 times greater in area, and result from the use of protrusionelements having converging sides such as shown at 76, 276, 376. Thechannels 20 are formed in the flexible graphite sheet 10 at a pluralityof pre-determined locations by mechanical impact at the predeterminedlocations in sheet 10 using a mechanism such as shown in FIG. 3comprising a pair of steel rollers 70, 72 with one of the rollers havingtruncated, i.e. flat-ended, prism-shaped protrusions 75 which impactsurface 30 of flexible graphite sheet 10 to displace graphite andpenetrate sheet 10 to form open channels 20. In practice, both rollers70, 72 can be provided with “out-of-register” protrusions, and a flatmetal plate indicated at 79, can be used in place of smooth-surfacedroller 72.

Indeed, the protrusion elements, such as protrusions 75, 175, 275, 375which impact surface 30 to form channels 20 can also have varying sizesor configurations, in order to impart differing characteristics toflexible graphite sheet 10 at differing locations, most preferably in aselected pattern. For instance, it may be desirable to have thepermeability of sheet 10 greater at certain locations than at others inorder to, e.g., facilitate water vapor dispersal from or throughflexible graphite sheet 10 where doing so may be needed or to equalizeor accommodate fluid or gas pressure along flexible graphite sheet 10.

This orientation of the expanded graphite particles 80 results inanisotropic properties in flexible graphite sheets; i.e. the electricalconductivity and thermal conductivity of the sheet being substantiallylower in the direction transverse to opposed surfaces 130, 140 (“c”direction) than in the direction (“a” direction) parallel to opposedsurfaces 130, 140. In the course of impacting flexible graphite sheet 10to form channels 20, as illustrated in FIG. 3, graphite is displacedwithin flexible graphite sheet 10 by flat-ended (at 77) protrusions 75to push aside graphite as it travels to and bears against smooth surface73 of roller 70 to disrupt and deform the parallel orientation ofexpanded graphite particles 80 as shown at 800 in FIG. 5. This region of800, adjacent channels 20, shows disruption of the parallel orientationinto an oblique, non-parallel orientation is optically observable atmagnifications of 100× and higher. In effect the displaced graphite isbeing “die-molded” by the sides 76 of adjacent protrusions 75 and thesmooth surface 73 of roller 70 as illustrated in FIG. 5. This reducesthe anisotropy in flexible graphite sheet 10 and thus increases theelectrical and thermal conductivity of sheet 10 in the directiontransverse to the opposed surfaces 30, 40. A similar effect is achievedwith frusto-conical and parallel-sided peg-shaped flat-ended protrusions275 and 175. As comparison, FIG. 4 is an enlarged sketch of a sheet offlexible graphite 110 that shows a typical prior art orientation ofcompressed expanded graphite particles 80 substantially parallel to theopposed surfaces 130, 140.

The perforated gas permeable flexible graphite sheet 10 of FIG. 1 areused as an electrode in an electrochemical fuel cell 500 shownschematically in FIGS. 6, 7 and 7(A).

FIG. 6, FIG. 7 and FIG. 7(A) show, schematically, the basic elements ofan electrochemical Fuel Cell, more complete details of which aredisclosed in U.S. Pat. Nos. 4,988,583 and 5,300,370 and PCT WO 95/16287(Jun. 15, 1995) and each of which is incorporated herein by reference.

With reference to FIG. 6, FIG. 7 and FIG. 7(A), the Fuel Cell indicatedgenerally at 500, comprises electrolyte in the form of a plastic e.g. asolid polymer ion exchange membrane 550; perforated flexible graphitesheet electrodes 10 in accordance with the present invention; and flowfield plates 1000, 1100 which respectively abut electrodes 10.Pressurized fuel is circulated through grooves 1400 of fuel flow fieldpate 1100 and pressurized oxidant is circulated through grooves 1200. Inoperation, the fuel flow field plate 1100 becomes an anode, and theoxidant flow field plate 1000 becomes a cathode with the result that anelectric potential, i.e. voltage is developed between the fuel flowfield plate 1000 and the oxidant flow field plate 1100. The abovedescribed electrochemical fuel cell is combined with others in a fuelcell stack to provide the desired level of electric power as describedin the above-noted U.S. Pat. No. 5,300,370.

One significant difference lies in the fact that the catalyst 600 isselectively loaded on the surfaces of the solid polymer exchangemembrane 550. In this way, the catalyst metal is only present in aselected pattern on membrane 550 rather than relatively uniformlydistributed thereon and, therefore, the amount of catalyst employed isminimized while maximizing the effectiveness of the catalyst. This isbecause catalyst is now only disposed on the surfaces of the membranewhere the walls 3 of channels 20 about membrane 550.

The operation of Fuel Cell 500 requires that the electrodes 10 be porousto the fuel and oxidant fluids, e.g. hydrogen and oxygen, to permitthese components to readily pass from the grooves 1400, 1200 throughelectrodes 10 to contact the catalyst 600 on the surfaces of themembrane 550, as shown in FIG. 7(A), and enable protons derived fromhydrogen to migrate through ion exchange membrane 550. In the electrode10 of the present invention, channels 20 are positioned to adjacentlycover grooves 1400, 1200 of the flow field plates so that thepressurized gas from the grooves passes through the smaller openings 60of channels 20 and exits the larger openings 50 of channels 20. Theinitial velocity of the gas at the smaller openings 60 is higher thanthe gas flow at the larger openings 50 with the result that the gas isslowed down when it contacts the catalyst 600 on the surface of membrane550 and the residence time of gas-catalyst contact is increased and thearea of gas exposure at the membrane 550 is maximized. This feature,together with the increased electrical conductivity of the flexiblegraphite electrode of the present invention enables more efficient fuelcell operation.

FIG. 8 is a photograph (original magnification 100×) of a body offlexible graphite corresponding to a portion of the sketch of FIG. 5.

The articles of FIGS. 1 and 5 and the material shown in the photograph(100×) of FIG. 8 can be shown to have increased thermal and electricalconductivity in the direction transverse to opposed parallel, planarsurfaces 30, 40 as compared to the thermal and electrical conductivityin the direction transverse to surfaces 130, 140 of prior art materialof FIG. 4 in which particles of expanded natural graphite unaligned withthe opposed planar surfaces are not optically detectable.

A sample of a sheet of flexible graphite 0.25 mm thick having a densityof 0.3 g/cc, representative of FIG. 4, was mechanically impacted by adevice similar to that of FIG. 3 to provide channels of different sizein the flexible graphite sheet. The transverse (“c” direction)electrical resistance of the sheet material samples was measured and theresults are shown in the table below.

Also, the transverse gas permeability of channeled flexible graphitesheet samples, in accordance with the present invention, was measured,using a Gurley Model 4118 for Gas Permeability Measurement.

Samples of channeled flexible graphite sheet in accordance with thepresent invention were placed at the bottom opening (9.5 mm diam.) of avertical cylinder (76.2 mm diameter cross-section). The cylinder wasfilled with 300 cc of air and a weighted piston (142 g) was set in placeat the top of the cylinder. The rate of gas flow through the channeledsamples was measured as a function of the time of descent of the pistonand the results are shown in the table below.

Flexible Graphite Sheet (0.25 mm thick; density = 0.3 gms/cc) 39channels per 250 channels per square cm-0.050 square cm-0.050 cm inchwide at top; wide at top; 0.0125 0.0175 cm wide at No Channels cm wideat bottom bottom Transverse 80 8 0.3 Electrical Resistance (micro ohms)Diffusion Rate- — 8 seconds 30 seconds Seconds

In the present invention, for a flexible graphite sheet having athickness of 0.075 mm to 0.375 mm adjacent the channels and a density of0.5 to 1.5 grams per cubic centimeter, the preferred channel density isfrom 160 to 465 channels per square cm and the preferred channel ;sizeis a channel in which the ratio of the area of larger channel opening tothe smaller is from 50:1 to 150:1.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible variations and modifications that will become apparent to theskilled worker upon reading the description. It is intended, however,that all such modifications and variations be included within the scopeof the invention that is defined by the following claims. The claims areintended to cover the indicated elements and steps in any arrangement orsequence that is effective to meet the objectives intended for theinvention, unless the context specifically indicates the contrary.

What is claimed is:
 1. A membrane electrode assembly comprising a pairof electrodes and an ion exchange membrane having opposed major surfacespositioned between the electrodes, at least one of the electrodes beingformed of a sheet of a compressed mass of expanded graphite particleshaving a plurality of transverse fluid channels passing through thesheet between first and second parallel, opposed surfaces of the sheetin a selected pattern, the channels being separated by walls ofcompressed expandable graphite, the walls formed on one of the opposedsurfaces abutting the ion exchange membrane, wherein the ion exchangemembrane has catalyst selectively loaded on or at least one of itsopposed major surfaces; and wherein the channel openings at the firstsurface are larger than the channel openings at the second surface. 2.The assembly of claim 1 wherein the compressed mass of expanded graphiteparticles is characterized by expanded graphite particles adjacent thechannels extending obliquely with respect to the opposed surfaces. 3.The assembly of claim 1 wherein the graphite sheet has a thickness ofabout 0.075 mm to 0.375 mm adjacent said channels and a density of about0.5 to 1.5 grams per cubic centimeter.
 4. The assembly of claim 1wherein the catalyst is loaded on or at both major surfaces of the ionexchange membrane.
 5. The assembly of claim 4 wherein the catalyst isloaded on or at a surface of the ion exchange membrane in a patterncorresponding to the pattern of the walls of the channels of the sheetof a compressed mass of expanded graphite particles abutting thatsurface of the ion exchange membrane.
 6. The assembly of claim 1 whereinthe configuration and/or size of the transverse fluid channels passingthrough the sheet between first and second parallel, opposed surfaces ofthe sheet varies in a selected pattern.
 7. An electrode for a protonexchange membrane fuel cell, comprising a sheet of a compressed mass ofexpanded graphite particles having a plurality of transverse fluidchannels passing through the sheet between first and second parallel,opposed surfaces of the sheet in a selected pattern; wherein the channelopenings at the first surface are larger than the channel openings atthe second surface.
 8. The electrode of claim 7 wherein the pattern ofchannels can be formed by selective placement of the channels,variations in channel density, variations of channel shape, orcombinations thereof.
 9. The electrode of claim 8 wherein the compressedmass of expanded graphite particles is characterized by expandedgraphite particles adjacent the channels extending obliquely withrespect to the opposed surfaces.
 10. The electrode of claim 7 whereinthe graphite sheet has a thickness of about 0.075 mm to 0.375 mmadjacent said channels and a density of about 0.5 to 1.5 grams per cubiccentimeter.
 11. The electrode of claim 7 wherein the graphite sheetcomprises walls lying between the transverse fluid channels thereof, thegraphite sheet having catalyst selectively loaded on at least some ofthe walls.
 12. A process for forming a membrane electrode assembly for aproton exchange membrane fuel cell, comprising forming channels in asheet of a compressed mass of expanded graphite particles having twoopposed major surfaces and combining the graphite sheet with the ionexchange membrane having catalyst selectively loaded on or at least oneof its opposed major surfaces; wherein the channels have openings on oneof the opposed surfaces that are larger than the openings in the otheropposed surface.
 13. The process of claim 12 wherein the channels areformed in the graphite sheet by a pressing force using flat-endedprotrusion elements.
 14. The process of claim 12 wherein the channelshave different configurations formed when using flat-ended protrusionelements of different shapes.
 15. The process of claim 14 wherein theprotrusion elements have decreasing cross-section in the direction awayfrom the pressing force to provide larger channel openings on the sideof the graphite sheet that is initially impacted.
 16. The process ofclaim 12 wherein the channels are formed using at least one patternedroller.
 17. The process of claim 12 wherein the graphite sheet compriseswalls lying between the transverse fluid channels thereof, the graphitesheet having catalyst selectively loaded on at least some of the walls.18. The process of claim 12 wherein the catalyst is loaded on or at atleast one of the opposed surfaces of the ion exchange membrane by aprinting process.
 19. An electrode for a proton exchange membrane fuelcell, comprising a sheet of a compressed mass of expanded graphiteparticles having a plurality of transverse fluid channels passingthrough the sheet between first and second parallel, opposed surfaces ofthe sheet in a selected pattern; wherein the graphite sheet compriseswalls lying between the transverse fluid channels thereof, the graphitesheet having catalyst selectively loaded on at least some of the walls.20. The electrode of claim 19 wherein the pattern of channels can beformed by selective placement of the channels, variations in channeldensity, variations of channel shape, or combinations thereof.
 21. Theelectrode of claim 20 wherein the compressed mass of expanded graphiteparticles is characterized by expanded graphite particles adjacent thechannels extending obliquely with respect to the opposed surfaces. 22.The electrode of claim 21 wherein the channel openings at the firstsurface are larger than the channel openings at the second surface. 23.The electrode of claim 21, wherein the graphite sheet has a thickness ofabout 0.075 mm to 0.375 mm adjacent said channels and a density of about0.5 to 1.5 grams per cubic centimeter.
 24. A process for forming amembrane electrode assembly for a proton exchange membrane fuel cell,comprising forming channels in a sheet of a compressed mass of expandedgraphite particles having two opposed major surfaces and combining thegraphite sheet with the ion exchange membrane having catalystselectively loaded on or at least one of its opposed major surfaces;wherein the sheet comprises walls lying between the transverse fluidchannels thereof, the sheet having catalyst selectively loaded on atleast some of the walls.
 25. The process of claim 24 wherein thechannels are formed in the graphite sheet by a pressing force usingflat-ended protrusion elements.
 26. The process of claim 25 wherein thechannels have openings on one of the opposed surfaces that are largerthan the openings in the other opposed surface.
 27. The process of claim26 wherein the channels have different configurations formed when usingflat-ended protrusion elements of different shapes.
 28. The process ofclaim 27 wherein the protrusion elements have decreasing cross-sectionin the direction away from the pressing force to provide larger channelopenings on the side of the graphite sheet that is initially impacted.29. The process of claim 24 wherein the channels are formed using atleast one patterned roller.
 30. The process of claim 24 wherein thecatalyst is loaded on or at at least one of the opposed surfaces of theion exchange membrane by a printing process.